In a general steam turbine, an inner casing is provided in an outer casing, a steam inlet is provided at an upper part thereof, a rotor is rotatably supported in a central part thereof, and rotor blades are fixed to the rotor in multiple stages. Besides, stator vanes are fixed in multiple stages to a turbine diaphragm ring supported in the inner casing, so that the rotor blades fixed to the rotor and the stator vanes are alternately arranged.
Therefore, when the steam enters the inner casing from the steam inlet at the time of operating the steam turbine, the steam blows out to the rotor blades via a partition wall through the stator vanes supported by the turbine diaphragm ring, to rotate the rotor and drive a power generator connected to the rotor.
The casing (outer casing, inner casing) that constitutes a turbine body of the steam turbine is divided into two parts, that is, upper and lower casings on a plane passing through a rotor, and is assembled by a flange and a bolt to improve the workability in insertion, assembly, and disassembly operations of the rotor. When the steam turbine is a low-pressure turbine, a lower part of the outer casing is connected to a steam condenser, and at the time of activating the steam turbine, the inner casing and the outer casing are evacuated to a vacuum state, thereby sucking the steam from the steam inlet into the casing (see Patent Document 1).
At this time, the outer casing deforms in such a manner that end plates forming a ceiling and walls of the outer casing are largely depressed inward, and thus the ceiling and walls need to be reinforced. As a reinforcing structure of the outer casing in the steam turbine, for example, there is a structure in which a plurality of ribs divided uniformly around a rotor shaft are bonded to the end plates of the outer casing by welding or the like, and a plurality of ribs are bonded to the lower-half end plates of the outer casing crosswise in a lattice-like arrangement by welding or the like. Furthermore, there is a structure in which a pipe stay is built inside an outer casing (see Patent Document 2).
In such a casing structure of such a steam turbine, a plurality of I-shaped ribs are arranged on an upper-half end plates of an outer casing radially around a rotor shaft, and an arrangement angle thereof is gradually increased as the angle changes from vertical to horizontal.    Patent Document 1: Japanese Patent Application Laid-open No. 2005-113721    Patent Document 2: Japanese Patent No. 3831617